This invention relates to electrical film capacitors.
Electrical film capacitors are known. One type is exemplified by the Aerofoil.RTM., Aerokraft.RTM., and non-microwave Aeropak.RTM. capacitors sold by Aerovox, Inc. These capacitors feature strips wound or folded together. The strips include a pair of electrodes separated by a solid dielectric spacer. A portion of the capacitor is porous so that it can wick a liquid dielectric, thereby drawing the liquid dielectric into the bulk of the capacitor and causing it to fill the spaces between the strips throughout the capacitor. Examples of suitable materials for the porous portion include Kraft paper. The dielectric spacer may act as the porous portion. The porous portion can also form part of one or both of the electrodes (e.g., in the case of metallized paper electrodes). Examples of liquid dielectrics which have been used in these capacitors include aromatic phthalate esters such as di-octyl phthalate.
Smooth metallized film capacitors are a second type of electrical film capacitor. They typically include smooth metallized thermoplastic strips wound or folded together. Such capacitors have as their electrode the metallized portion of the strip and as the dielectric spacer the thermoplastic portion of the strip. These capacitors, however, lack a porous portion that can wick the liquid dielectric and draw it into the bulk of the capacitor. As a result, the liquid dielectric (even under vacuum impregnation) at best penetrates only the end regions of the capacitor, and even then only to a small degree (if at all). Consequently, the spaces between the strips in the interior of the capacitor (and indeed throughout most of the capacitor) are devoid of liquid dielectric.
Various aromatic and non-aromatic liquid dielectrics have been used to fill metallized film capacitors over the years. Ross et al., U.S. Pat. No. 3,855,508, for example, describes certain adipic acid esters (specifically, di-isooctyl adipate and di-isodecyl adipate) for this purpose. Tracy et al., U.S. Pat. No. 2,947,927 also describes certain adipic acid esters for use as liquid dielectrics in metallized film capacitors (specifically, di-isobutyl adipate, di-hexyl adipate, di-isooctyl adipate, and di-isohexyl adipate). Examples of aromatic liquid dielectrics include phthalate esters (e.g., di-octyl phthalate) and biphenyls (e.g., monoisopropyl biphenyl and halogenated biphenyls).